Keggin-Type Tridecanuclear Europium-Oxo Nanocluster Protected by Silsesquioxanes

The Keggin structure, known as an archetypical polyoxometalate (POM) structural motif, has widened up our fundamental understanding of the growth behavior of natural metal oxide minerals. However, devising a strategy for synthesis of such species consisting of pure-metal ions remains a great challenge. Different from those six-coordinated early transition metal ions in Keggin POMs, lanthanide prefers higher coordination numbers, making the construction of lanthanide-Keggin structure more difficult. Here, we report the unprecedented luminescent Keggin-type Eu13 nanocluster constructed through a facile solvothermal method using the high-symmetry macrocyclic tetrasilane (Ph4Si4O8)4– as the capping ligand. This protocol demonstrates an important proof of concept for assembly of metallasilsesquioxanes. The structure shows a rare polyoxometalate-Eu-oxo-core@SiO2-shell@organic-shell (Eu13) consisting of a central EuO8 cube, surrounded by 12 edge-sharing EuO7 monocapped trigonal prisms. It is the highest-nuclearity lanthanide–silsesquioxane nanocluster. Electrospray ionization mass spectrometry reveals that Eu13 is stable in solution except for some labile terminal dimethylformamide on the ligand shell, which endows Eu13 with a high catalytic activity for CO2 cycloaddition reaction with small epoxides under ambient conditions. Eu13 also exhibits intense red luminescence upon visible light excitation (466 and 535 nm). The photophysical and photoelectric properties of the drop-casting film of Eu13 indicate its great potential in optoelectric device applications.